In certain machine control circuits and the like, a transistor switch is turned on or off by a control voltage applied through a movable contact switch. Movable contact switches are susceptible to contact bounce when the contacts are closed and a momentary or transient pulse is produced by the bouncing engagement of the contacts before the contacts reach a stable, closed state to produce the steady state control voltage for the switching transistor. The contact bounce pulse, even though of very short duration, may be effective to switch the transistor and thus produce an undesired or false transistor output signal. In the system of the above-referenced patent, the transistor ouput signal is a logic signal used in self-checking and a false signal has the effect of indicating a fault in the system which results in shut-down of the machine. It is desired to provide means for eliminating this false signaling.